The x-ray and/or tomography images obtained by x-ray image recording apparatuses, in particular computed tomographs, may comprise various image artifacts. One type of image artifact is used such that the measured object is not completely captured during the measuring process in terms of its geometric extension. Part of the object under measurement is positioned outside of the field of view and is in this manner truncated, so to speak, in respect of the image obtained therefrom. The image artifacts resulting herefrom can be referred to below as aliasing artifacts overlapping the field of view. They play an essential role particularly in computed tomographs, since a three-dimensional image obtained by means of back projection is frequently based on a plurality of projection images, not all of which capture the object to be measured wholly or completely. The object is not constantly completely within the field of view, namely during the measuring process.
This unwanted data shortening may be meaningful in the case of all computed tomographical scan apparatuses, but nevertheless plays a significant roll particularly with flat panel computed tomographs (see “W. A. Kalender and Y. Kyriaku. Flat-detector CT. Eur Radiol. (11):2767-79, 2007”). With flat panel detector computed tomographs, the view field and/or field of view of the detector which can be captured during the measurement only amounts to approximately 20-25 cm in terms of diameter. This restriction makes the prevention of aliasing artifacts overlapping the field of view almost impossible. Aliasing artifacts overlapping the field of view significantly impair the quality of a resulting x-ray and/or tomography image. The artifacts not only herewith appear in the vicinity of the image edge, but instead also influence central areas of the recorded image.
Aliasing artifacts overlapping the field of view would then not occur for instance if the x-ray radiation was not attenuated at all border areas of the field of view. A defined transition in respect of the absorption values to zero then results. If this transition is however not correctly given, this results during the computed tomography recordings particularly after filtered back projection (see for instance “A. C. Kak and M. Slaney. Principles of Computerized Tomographic Imaging. IEEE Press, 1988”, “L. A. Feldkamp, L. C. Davis, and J. W. Kress. Practical cone-beam algorithm. J. Opt. Soc. Am. A, 1(6):612-619, 1984”) in the effect that aliasing artifacts overlapping the field of view appear and an apparent increase in the x-ray radiation attenuation values to the image borders is observed. A pale white ring appears in the computed tomography image beyond the border of the field of view. Strip-like artifacts also result outside of the actual field of view area.
Aliasing artifacts overlapping the field of view are generally suppressed such that image areas at the edge of the field of view, to which attenuation values greater than zero are assigned, are extrapolated such that a smoothed value curve is produced toward the x-ray absorption value zero. According to a known method, the truncation areas are extrapolated in the computed tomography projection images used for the back projection onto an attenuation value of zero and it is only then that the filtered back projection is implemented. Within the scope of this extrapolation method, objects are approached for instance by means of a water cylinder (see “Hsieh J, Chao E, Thibault J, Grekowicz B, Horst A, McOlash S and Myers T J, 2004, A novel reconstruction algorithm to extend the CT scan field-of-view Med. Phys. 31, 2385-91”). The patient as a whole can also be approximated as a water ellipsoid, so that in this manner data exists for the extrapolation (see “Maltz J S, Bose S, Shukla H P and Bani-Hashemi A R, 2007, CT truncation artifact removal using water-equivalent thicknesses derived from truncated projection data Proc. IEEE Eng. Med. Biol., Soc. 2007. 2907-11”). A square extrapolation is for instance known from “Sourbelle K, KachelrieB M and Kalender W A, 2005, Reconstruction from truncated projections in CT using adaptive detruncation Eur. Radiol. 15, 1008-14”, while a so-called sinogram interpolation is described in “Zamyatin A A and Nakanishi S, 2007, Extension of the reconstruction field of view and truncation correction using sinogram decomposition Med. Phys. 34, 1593-604”. Further extrapolation methods are known from the following publications: “Janoop K P and Rajgopal K, 2007, Estimation of missing data using windowed linear prediction in laterally truncated projections in cone-beam CT Proc. IEEE Eng. Med. Biol. Soc. 2007, 2903-6”, “Starman J, Pelc N, Strobe N and Fahrig R, 2005, Estimating 0th and 1st moments in C-arm CT data for extrapolating truncated projections Proc. SPIE 5747, 378-87” and “Sourbelle K, KachelrieB M and Kalender W A, 2005, Reconstruction from truncated projections in CT using adaptive detruncation Eur. Radiol. 15, 1008-14”.
The methods known from the prior art have the objective of improving the image quality within the field of view area, but nevertheless impairing an image modification or quality improvement outside of the field of view area. In the event that several border areas are truncated in the computed tomography projection images, further serious disadvantages result. With the majority of methods, at least one non-truncated projection image is needed in order to ensure the fulfillment of a consistency criterion. A conversion from 3D into 2D data is frequently extremely time-consuming. Extremely shortened data records, such as are the rule with flat panel computed tomographs, cannot be overcome by the usual methods with respect to the aliasing artifacts overlapping the field of view. In addition, anatomical information is frequently lost. The contour of a patient is generally not correctly reproduced, which hampers a treating physician during an operation for instance, when navigating instruments in the body of the patient with the aid of the computed tomography image.